1. Field of the Invention
This invention relates to the coupling of anionic polymers and to the hydrogenation of such coupled polymers to make a polymer composition containing low levels of uncoupled polymer and a mixture of linear and radial polymers.
2. Background of the Art
The coupling of lithium-terminated polymers is a process known in the art. In accordance with this known process, a lithium-terminated polymer is treated with a compound having two or more functional groups containing two or more reactive sites capable of reacting with the carbon-lithium bonds of the lithium-terminated polymer. In many cases the multifunctional coupling agent thereby becomes a nucleus for the resulting structure. From this nucleus long chain polymeric branches radiate and such coupled polymers have specific properties that render them useful for particular applications.
Linear polymers are formed by employing coupling agents having two reactive sites. For example, one coupling agent employed in making linear polymers is methyl benzoate as disclosed in U.S. Pat. No. 3,766,301. Radial polymers are formed by employing coupling agents having more than two reactive sites. Examples of such coupling agents include among others silica compounds, including silicon tetrachloride and alkoxy silanes—U.S. Pat. Nos. 3,244,664, 3,692,874, 4,076,915, 5,075,377, 5,272,214 and 5,681,895; polyepoxides, polyisocyanates, polyimines, polyaldehydes, polyketones, polyanhydrides, polyesters, polyhalides—U.S. Pat. No. 3,281,383; diesters—U.S. Pat. No. 3,594,452; methoxy silanes—U.S. Pat. No. 3,880,954; divinyl benzene—U.S. Pat. No. 3,985,830; 1,3,5-benzenetricarboxylic acid trichloride—U.S. Pat. No. 4,104,332; glycidoxytrimethoxy silanes—U.S. Pat. No. 4,185,042; and oxydipropylbis(trimethoxy silane)—U.S. Pat. No. 4,379,891.
The production of styrenic block copolymers such as S-E/B-S triblocks by coupling has a number of process advantages over sequential polymerization, such as better control over the styrene block size and lower viscosity during polymerization. However, the inevitable presence of un-coupled arms can limit product performance. Diblock contamination can greatly reduce tensile strength and related properties in a triblock copolymer or compound thereof. S-E/B-S polymers for use in applications such as highly-oiled compounds cannot afford to sacrifice in this area. It is generally difficult to achieve coupling efficiencies of better than 90%. While coupling efficiencies on the order of 90% can be achieved by reaction with m-divinylbenzene, the resulting products are high molecular weight “star” polymers. Although the melt viscosity of such a polymer is much lower than a linear product of the same total molecular weight, it is much higher than that of the corresponding triblock that would be prepared by coupling two of the diblock arms.
It would be highly desirable to identify a coupling agent that gives greater than 90% of a substantially linear product, or, at least, a mixture of linear and radial polymers. It would be particularly advantageous if coupling efficiencies approaching 95% could be obtained in systems that result in a butadiene microstructure suitable for hydrogenation to give a saturated rubber block. Such products would be expected to have properties that are comparable to sequentially polymerized S-E/B-S polymers, which are often contaminated with some diblock due to a variety of side reactions. It would also be highly advantageous if residual coupling agent or its by-products were found to have no adverse affect on the activity of the hydrogenation catalyst.